We are interested in the reciprocal coupling of core metabolism and other cellular processes. Metabolic and nutrient status elicit substantial effects on many aspects of cellular biology, including cell growth, cell division, protein synthesis and many others. Conversely, many cellular signaling pathways exert control on important metabolic decisions. Our broad goal is to begin to understand how this crosstalk occurs in normal situations and how its impairment is involved in disease states. Specifically, we are studying the regulation and function of a protein we think is one important mediator of this crosstalk, PAS kinase.

PAS kinase regulation
PAS kinase is a serine/threonine kinase conserved from yeast to humans. In addition to a canonical kinase catalytic domain, it contains a regulatory PAS domain. We have found that the PAS domain of PAS kinase specifically interacts with and inactivates the kinase catalytic domain (Rutter, et al. 2001; Amezcua, et al. 2002). NMR-based studies, done in collaboration with Dr. Kevin Gardner at UT Southwestern, have shown that the PAS kinase PAS domain is also capable of binding specific small organic compounds (Amezcua, et al. 2002). We have evidence that such PAS-binding compounds might be able to disrupt the PAS domain-kinase domain interaction and thereby stimulate kinase activity.
A major focus of our laboratory is to identify the endogenous molecule(s) responsible for derepressing PAS kinase catalytic activity in vivo via interaction with the inhibitory PAS domain. Preliminary studies have identified an activity from fractionated bovine brain that is capable of substantially stimulating PAS kinase activity. We are currently developing methodology to purify this PAS kinase activator. These studies will feed into our ongoing collaboration with Kevin Gardner to understand the mechanistic basis for signal sensing and transduction by this versatile PAS domain.

Regulation of mammalian glycogen accumulation
The second primary focus of our laboratory is to understand the role of PAS kinase in controlling glycogen synthesis in mammals. The storage of glycogen in muscle and liver is the primary means whereby the body disposes of excess blood glucose, and 

impairment of this process is a major contributor to glucose intolerance, insulin resistance and diabetes. The study of factors that block glycogen deposition is important not only for understanding diabetes pathogenesis, but also for development of potential targets for anti-diabetic therapy. In collaboration with Dr. Peter Roach, we found that human PAS kinase phosphorylates mammalian glycogen synthase and potently represses its catalytic activity in vitro. Preliminary studies in cellular models of glycogen accumulation show that PAS kinase overexpression leads to a decrease in glycogen deposition. We are further characterizing this phosphorylation event biochemically. In addition, we are involved in a program to understand the role of PAS kinase in regulating glycogen deposition in vivo using genetically altered mice.

PAS kinase function in yeast
We are also continuing our earlier work on the effects of phosphorylation by PAS kinase on five interesting yeast proteins (as described in our 2002 Cell paper). These include three proteins involved in protein synthesis. We have more information on the other two, which are the enzymes that catalyze the final two steps in glycogen synthesis. Interestingly, while we have genetic evidence for both of these proteins that phosphorylation is important in controlling their activity, in neither case does it affect their activity directly. Phosphorylation must, therefore, be controlling the ability of these proteins to interact with other molecules that either properly localize or coordinate their enzymatic activity. We are using a wide array of approaches to study all aspects of these phenomena.